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I'm building a boat shed, adding on to an existing shed (which has corrugated metal over purlins, no underlayment). I'm planning on corrugated 26 ga. galvanized roofing. But I've read a lot about condensation and am considering underlayment. The only reason I'm considering it is that I don't want rusty drips on my boat. BTW I'm in upstate NY, so perhaps the biggest moisture concern is snowmelt in the Spring.
Rafters are horizontal to the slope, and it's a low slope roof, 1.5:12. I'm planning to attach it directly to the 2x10 rafters which are 16" OC, with the neoprene washer screws. Not sure if they're galvanized screws, but they say they have an "OxcySeal" coating on them.
However, I've never had any condensation issues on the 50-year-old existing shed (which has pretty heavy guage on it, maybe 24 or heavier). I do get some dripping during snowmelt but that's mostly due to holes in the roof.
So I think that pretty much shows that underlayment probably isn't really needed... but thought I'd check with the Experts first. Will an underlayment like Tiger Paw or Triflex XT really help with condensation/snowmelt? And even if I put an underlayment on, won't the moisture work it's way under the seams in the underlayment anyway?
I understand your situation and concerns ... and do not think that underlayment is the answer. It would just transfer any resulting condensation to the underside of itself rather than the roofing. Here's a product that is really designed for your situation: http://dripstop.net
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